The invention relates to a method for producing small and micro-sized ceramic parts by lithography. A shaping or working (for example, the micro-structuring) of ceramic materials is practically impossible after sintering because the material is then too hard and too brittle.
It is therefore necessary to develop machining processes for the ceramic base material before sintering while it is present as a lower viscosity organic or inorganic solvent or as a wax or thermoplastic material matrix. Ceramic microstructures with structural details  less than 100 xcexcm and an aspect ratio (ratio of height of structure to the lateral dimension) greater than 1 can be manufactured therefore only with relatively expensive processes. The LIGA process (x-ray depth lithography, galvanic forming and molding), which was developed at the Forschungszentrum Karlsruhe, has, until recently, permitted only the direct manufacture of microstructures from metal and plastic materials by the use of PMMA resist materials. These plastic microstructures can be copied as lost negative molds in an additional process step by means of foil casting, stamping, dross pressing or centrifugal molding into ceramics, [1]:(J. Ritzhaupt-Kleissel, Keramik in der Mikrotechnik-Werkstoffe, Verfahren and Anwendungen, 3. Statuskolloquim des Projektes Mikrosystemtechnik, 02./03.04.1998, Karlsruhe). Further development of the ceramic injection molding for the requirements of micro-forming procedures (machine techniques, powder size, binder sytems etc.) lead to ceramic hot casting (waxes as binders) or respectively, to ceramic micro-injection molding (thermoplastics or respectively, thermoplastics/wax mixtures as binders). [2]: Stadel, H. Freimuth, V. Hessel, M. Lacher, Abformung kermischer Mikrostukturen durch die LIGA-Technik, Keram. Z. 48(12), 1112ff, (1996); [3]: Plotter, T. Benzler, J. Hausselt, R. Ruprecht, MIM und CIM-Neue pulvertechnologische Verfahren in der Mikrotechnik, 3. Statuskolloquium des Projektes Mikrosystemtechnik, 02./03.04.1998, Karlsruhe.
The organic binder must be removed at raised temperatures and the remaining powder structure must be sintered to form stable micro-structured parts. Because the ceramic powder mixture (feedstock) must be capable of flowing during hot casting or injection molding the ceramics content can be only relatively low (50-70% vol.). As a result, the parts shrink during sintering by 30-50% of the mold which must be taken into consideration during design. The use of a pre-ceramic polymer material as binder which, after forming of the part, is pyrolized by heating to raised temperatures to be converted to a ceramic reduces the shrinking during sintering [2]: Stadel, H. Freimuth, V. Hessel, M. Lacher, Abformung kermischer Mikrostukturen durch die LIGA-Technik, Keram. Z. 48(12), 1112ff, (1996). However, the pre-ceramic polymers have generally a chemical instability to moisture in the air so that the injection molding and casting must be performed under controlled conditions [4]: Hessel, H. Freimuth, M. Stadel, M. Lacher, W. Ehrfeld, Fabrication of Complex Ceramic Microstructures from Powder Ceramics and Preceramic Polymers, Tagungsband Micro Materials (Micro Mat ""97), 16-18.04.1997, Berlin, 370ff.]
All these processes further require a micro-structured master mold of metal (molding appliance) or of plastic (lost mold)xe2x80x94in contrast to the process according to the invention. In the last-mentioned case, the master mold is subjected to the pyrolizing step with the ceramic component part disposed thereon.
In the microelectronic and micro-technical art, radiation sensitize plastic materials and polymers are known which are used in lithographic procedures as photo-resists and whose solubility is changed by the irradiation. The manufacture of microstructures by lithographic procedures requires in principle four processing steps that is the coating by lacquer (spin-coating) the imaging exposure to electromagnetic (actinic) radiation through a mask provided with a structure, the developing, and, in addition, generally a subsequent thermal treatment at raised temperatures for the removal of residual solvents in the polymer film and to improve the adhesion to the substrate. Generally, during the radiation exposure, polymerketes are destroyed and, consequently, the molecular weight of the polymer is reduced whereby the solubility of the polymer which was originally insoluable in corresponding organic solvents has an increased solubility with respect to the material which was not exposed to the radiation. After the development procedure, the non-exposed polymer which converts the two-dimensional relief picture of the mask to a three-dimensional polymer structure, remains on the substrate. With such positive resists, homogeneous layers with a thickness of over 100 xcexcm can be made. The irradiation of negative resist materials leads to a transverse netting by polymerization or, respectively, polycondensation of the polymers or, respectively, oligomers. The solubility is substantially reduced thereby. The development with a solvent results therefore in an inverse mask structure since the unexposed areas of the resist are dissolved. With this method, plastic microstructures can be made lithographically in a direct way.
Pre-ceramic polymers are used for example in the manufacture of ceramic filters such as silicon carbide, silicon nitride, silicon carbon-nitride, and others. [5]: Vaahs, M. Brxc3xcck, W. D. G. Bxc3x6cker, Polymer-derived Silicon Nitride and Silicon Carbon nitride Fibers, Adv. Mater. 4(3), 224ff (1992). The treatment of the pre-ceramic polymers by moist air or at raised temperatures (below 500xc2x0 C.) results in a transverse netting and, as a result, in an insolubility in solvents. This is followed by a conversion to the ceramic state at temperatures of 1000-1200xc2x0 C. Pre-ceramic polymers may also be hardened by electromagnetic irradiation such as UV radiation, x-radiation or electron radiation. The UV induced transverse lattice-like polymerization of polysilicones in an oxidizing atmosphere (air, oxygen) at a temperature of less than 200xc2x0 C. with the use of a photo-mask for producing a structure is used for example for producing a temporary protective layer (thickness less than 1 xcexcm) on semiconductor elements or similar devices [6]: JP 05 088373]. Ceramics made in accordance with this oxidizing process have a relatively low mechanical and thermal stability because of a relatively high oxygen content.
EPA-A 0510 872 discloses a process for converting silicon-containing polymers to ceramics by pyrolysis. However, stable small and micro-parts can not be produced alone by pyrolysis.
It is the object of the present invention to provide a method by which ceramic structures with high thermal and mechanical stability can be produced.
In a method for the manufacture, by means of lithography, of small and micro-parts consisting of ceramics, a pre-ceramic silicon containing polymer layer including at least one of boron-, carbon-, phosphorus-, oxygen-, nitrogen-, and hydrogen- atoms is deposited on a highly temperature resistant substrate and then dried at room temperature; the layer is then exposed in an image pattern to electromagnetic radiation and then developed in an organic solvent to remove the non-exposed areas. The parts are then pyrolyzed at more than 900xc2x0 C. and then sintered at a temperature of at least 1600xc2x0 C. to form a ceramic structured layer on the substrate.
The method according to the invention permits the direct manufacture of ceramic microstructures from pre-ceramic polymers without the need for an intermediate molding step by x-ray depth or, respectively, UV lithography. The processing steps described below are performed at room temperature (25xc2x0 C.):
Spin coating: The pre-ceramic polymer or, respectively, a solution thereof in a solvent (for example, toluene, xylol) is spun onto a high-temperature resistant substrate (for example, 4xe2x80x3 aluminum oxide wafer). The desired layer thickness can be adjusted by way of the spinning conditions (start up ramp, speed). Subsequently, the film needs to be dried in order to remove the residual solvent. The film must not be baked at raised temperatures. The pre-ceramic polymer is applied to the substrate in pure form, that is, without solvent or as a solution in a solvent by spin coating, spin casting or painting.
Exposure: Highly parallel x-ray or respectively, UV radiation with a wavelength of less than 400 nm and preferably between 300 and 400 nm can be used for the irradiation of the polymer film through a lithographic mask. These highly parallel x-rays can be generated for example by a synchrotron. The preferred wavelength is smaller than 1 nm.
Development/etching: After exposure the unexposed areas of the film are removed by an organic solvent of a suitable polarity (for example, acetone, chloroform, dioxan, isopropanol, toluene), whereby the unexposed areas of the pre-ceramic polymer are dissolved by the solvent. It is important in this connection that the solvent is highly selective.
Pyrolysis: The undissolved pre-ceramic polymer is subjected, under a protective gas cover or a reaction gas cover, (helium, organ, nitrogen, ammonia) or in a vacuum, to a pyrolysis between 900 and 1200xc2x0 C. for 0.5 to 2 hours to convert to a ceramic.
Sintering: A stable body is obtained after a sintering procedure at temperatures of up to 2000xc2x0 C. preferably 1600-2000xc2x0 C. for 0.5 to 2 hours.
These process steps combine the excellent structuring capabilities of polymers with the advantageous mechanical thermal and chemical properties of the functional ceramic and represents a new direct method for the manufacture of ceramic microstructures. Highly temperature resistant and chemically inert ceramic micro parts for application for example in the chemical micro-engineering field (ceramic reactors, mixers, etc) can be manufactured directly and relatively inexpensively by lithographic procedures. In addition, the procedures described above permit rapid prototyping of micro-structured ceramic components since CAD data can be converted directly by means of lithographic procedures to ceramic prototypes.
The invention will be described below in greater detail on the basis of three examples.